The Colorado Learning Disabilities Research Center and Brian Byrne's research group at the University of New England in Australia will collaborate to assess genetic and environmental influences on the early development of reading and attention, and will attempt to identify the specific psychological processes that mediate these influences. We propose to recruit and test 340 twin pairs in Australia at preschool age to minimize contamination of our measures with reading experience, and to retest the twins at the end of kindergarten, first and second grade, to monitor growth during this critical period of early reading development. Project V will use the same measures with the Australian twins as those used in a companion project to be conducted with twins in Colorado under a new 5-year R01 grant from NICHD (608 pairs), and a smaller project currently underway in Australia, funded by the Australian Research Council (110 pairs). The Australian twins, combined with the Colorado twins (1,058 pairs total), will substantially increase statistical power for exploring the genetic and environmental etiology of both high and low performance in this large epidemiological sample. The proposed epidemiological sample and developmental/longitudinal design beginning in preschool will complement cross-sectional research with older school- history RD- and/or ADHD-selected twins studied in projects I-IV.